High efficiency klystron technology. I. Syratchev, CERN High Efficiency International klystron activity 2 2 J. CAI, CERN C. Marrelli, ESS A. Baikov, MUFA D. Constable, Lancaster U V. Hill, Lancaster U G. Burt, Lancaster U R. Kowalczyk, SLAC Since 2013

‘Simple” modulator breakdown cost model High efficiency! How high it could/should be!? Personal outlook and challenges in efficiency reach with single beam klystron amplifiers. ‘Simple” modulator breakdown cost model Efficiency impact on investment cost ~Power ~Voltage Efficiency, % Normalized cost Fixed RF power and beam perveance -16.3% -2.8% Installed cooling capacity Normalized volume -60% Klystron lifetime ≈𝑒𝑥𝑝 −𝐼 3.4 × 𝑉 −0.5 10% Normalized hours Common tubes, 95% of the market We still might do it? New bunching technology Challenge (a.u.) Ultimate(space charge – free) limit Reducing perveance ‘best’ existing tube Multi Beam technology. Reduced voltage and perveance. Cost and efficiency. Gated Cathode (triode gun). No HV switches. Modulator cost and efficiency. SC or PPM solenoid. Overall efficiency. Complimentary technologies MBK Depressed collector Efficiency, %

High efficiency! How high it could/should be!? 100 MW Efficiency impact on operation cost FCC e+e- at 50 Euro/MWh and 5000 hours/year: 9.3 M Euro 10.7 M Euro Excessive electricity bill, M Euro 1 year Efficiency, % 37 MW, more than twice the overall cryogenic power needs Pulsed, 0.7 GHz, 92 MW

New/advanced technology CLIC 20 MW L-band Multi-beam pulsed klystron. 20 MW, 50 Hz, 150 sec 150 kW 180 kV Modulator (=0.9) HV transformer Energy storage Total = 0.62 switch Cathode RF circuit (=0.7) Collector 60 KW Lower (<60kV) voltage: - 40 mini-cathodes No oil tank (cost) Shorter tube (cost) Faster switching (efficiency/cost) Gated mini-cathode: No switches (cost) Modulator efficiency ~1.0 (+) Improved stability 150 kW + 88 kW Solenoid 4 KW New klystron RF circuit (=0.9) (+) Reduced Collector dissipation (16 kW) Depressed collector (=0.5) Permanent Magnets: - No power consumption - Potential cost reduction Vs. SC solenoid: - More expensive solution New/advanced technology Total = 0.95 Power from grid: 290 190 MW

Recommended by industry State of the art. Commercial MBK (low perveance) tubes with high efficiency. After many decades of development the klystron technology was considered to be saturated. The experimental results from hundred’s of different devices have shown that higher efficiency is associated with lower perveance. Accounting for technological and cost reasons (K>0.2), the 75% efficiency was expected to be the utmost limit. MBK TH1803 10 beams 21 MW 1.0 GHz Klystron efficiency vs. perveance Recommended by industry Optimised at CERN 73.5% RF measurements with directional coupler. 23.04.2017

Bunch saturation issues. COM tubes with different numbers of stages: Bunch saturation issues. Optimised by 1D code KlypWin Reaching high efficiency requires that ALL the electrons entering the output cavity should populate the bunch (useful part of RF period) leaving anti-bunch empty. K=0.26 bunch anti-bunch To saturate the bunch, peripheral electrons should receive much stronger relative phase shift than the core electrons: non monotonic bunching. In the COM method the Core of the bunch experiences periodical Oscillations due to the space charge forces, whilst the peripherals approach the bunch centre monotonically. COM method requires very long length to achieve full bunch saturation.

Example of the ‘fully’ saturated bunch in COM tube (Tesla 2D code) Bunch saturation issues. Example of the ‘fully’ saturated bunch in COM tube (Tesla 2D code)

High efficiency klystrons. New bunching technologies on one slide. Core Oscillations Method (5.75 m) Bunching Alignment Collecting, 2.44 m 133.8 kV, 12.55 A, 1.4 MW at 0.8 GHz, 80(+)% COM Core Stabilization Method BAC High Efficiency International klystron Activity CSM_2L3B3, 1.88 m CSM igor.syratchev@cern.ch CSM_23B1, 1.72 m

From the klystron text book: Power conversion efficiency issues. From the klystron text book: “For the bunch entering output cavity, the good bunching (i.e., a high fundamental harmonic of the beam current) can only yield high conversion efficiency when the energy spread in stream is small.” Example: power extraction efficiency from the bunched beam with I1/I0=1.75 and E min=E0 (monochromatic bunch) at the output cavity entrance. The beam and cavity parameters were taken from FCC COM tube. = 1 2 𝐼 1 𝐼 0 𝐸 𝑚𝑖𝑛 𝐸 0 1 2 (1) Output cavity F/Q Loaded phase space. Fully saturated bunch 1D simulations with CERN in-house code (J. Cai) Unstable area: bouncing/ reflected electrons Zoom 85.3% Equation (1) F0 Power conversion efficiency Bunch deceleration in the output cavity F=1.00025xF0 Fully saturated bunch Simulations (J. Cai) 85.3% 83.3% accelerated electrons Modulation depth, I1/I0 In a ‘classical’ approach, the highest power conversion efficiency with fully saturated bunch does not exceed 87%. Ez in the cavity Gaussian ‘classical’ bunch Fully saturated ‘rectangular’ bunch

Ultimate high power conversion efficiency conditions: Power conversion efficiency issues. quick optimisation 89.4% I1/I0=1.75 Ultimate high power conversion efficiency conditions: To avoid migration of the electrons from bunch to anti-bunch during deceleration in the output cavity, the incoming bunch with non-zero length should have certain velocity dispersion: Congregated Bunch, when the leading electrons are slower than the tailing ones. For the full deceleration, the congregated bunch should be gradually transformed at the cavity exit into monochromatic bunch with least velocity. bouncing electrons Equation (1) congregated Power conversion efficiency saturated ‘classical’ Modulation depth, I1/I0

Harmonic current (bunch length) at different radii. Power conversion efficiency. Limiting factors. Example of HE CSM tube (full 3D CST simulations). Bunch Radial Stratification Efficiency 80% BRS is an effect when the bunch length shows radial dependence. It appears as a result of intrinsic radial imbalance between cavities RF impedance and the space charge forces of the beam. In 2D simulations this effect is the major source of reflected electrons generation that are not predicted in 1D simulations. We are not inefficient (2D vs. 1D), we simply cannot go higher – the reflected electrons collapse power generation. Bouncing (re-accelerated) electrons Harmonic current (bunch length) at different radii. Applegate diagrams for the electrons emitted at different radius: r=0 r=0.5 R beam r=0.9 R beam

BRS effect is almost mitigated! COM tube. 8_04_XX series of 10 optimised tubes. Frequencies scattering Klys2D (Thales), 81.6% MAGIC (PIC) 8_04_08; Eff. = 84.62% Drifts scattering Cavity 7 Volts Replacing cylindrical beam by hollow beam efficiency is further increased up to 86% Cavity 8 Volts 08_04_08 BRS effect is almost mitigated! 08_04

85.7% CSM tube. Second Generation. Tube length 1.72 m at 0.8 GHz. Saturation curve (MAGIC 2D) 85.7%

We have developed special procedure (GSP/PSP) which allows to scale the frequency, power, perveance, voltage, number of beams etc., and to preserve the bunching quality of the original tube: GSP/PSP 0.8 GHz, 133.9 kV, 12.551 A, K=0.263 0.704 GHz, 110 kV, 15.272 A, K=0.42 Length 1.4 m Length 1.72 m Expected efficiency reduction (AJDisk) 1.9%.

High efficiency klystron development, if being mapped over the three HEP pillars shown by Frédérick Bordry, is going from right to the left. It is moving from Study to the Project. We are convinced now that technology is ready to go for its first prototyping stage. The first meeting with Industry was help at CERN on April 24, 2017. We are planning that in close collaboration with Industry the full design study will be finished by November 2017. Followed by technical design, fabrication and testing of the first prototype late in 2018.

The choice of bunching technology may drive the applicable frequency range and multi-beam options (cost/performance): L-band S-band C-band X-band Kladistron CSM/modest MBK Medical/industrial CLIC, klystron based X-band FEL LHC,FSS,ESS,ILC BAC/MBK 1/6 MW 5-10 MW, <60kV COM/single beam CLIC, TBA High perveance MBK HE tubes (!?) 50+ MW 10-20 MW

Thanks for your attention! I hope I was efficient. High Efficiency International klystron activity 2 2 J. CAI, CERN C. Marrelli, ESS A. Baikov, MUFA D. Constable, Lancaster U V. Hill, Lancaster U G. Burt, Lancaster U R. Kowalczyk, SLAC Since 2013